Increasingly many critical applications use information supplied by constellations of navigation satellites (Global Navigation Satellite System or GNSS), either the position signal itself, or its time reference, or both. Such is the case of military applications, but subject to use by authorities accredited by the government controlling this constellation, the latter can use protected frequencies for which the signal benefits from a guarantee of authenticity. The applications of the civilian domain cannot as a general rule obtain such accreditations, even when they are very critical, as is the case with air navigation, maritime navigation or civilian security. They must therefore use the open signals, whose frequencies, wave forms and codes are known. These signals can therefore be counterfeited without excessive difficulty with a malign intent. Validating the authenticity of the navigation signal is therefore an issue that is critical in itself. “Spoofing” techniques (“spoofing” being the most commonly used terminology) have been developed, as have therefore “anti-spoofing” techniques.
A first category of conventional anti-spoofing methods comprises those consisting in correlating the receiver location information obtained from the GNSS signal with information originating from an external source reputed to be non-counterfeit (terrain mapping, altitude, inertial sensors, path travelled relative to an authenticated reference, etc). These methods are not however adequate, taken in isolation, for the level of certification imposed for certain applications embedded on aircraft such as the terrain collision avoidance system, which presupposes the integrity of each of the components of the system.
Anti-spoofing methods of a second category have therefore been developed to authenticate a GNSS signal on the basis of its intrinsic characteristics perceived by the receiver, in particular the radiation patterns of its antenna. The objective of these methods is to be able to eliminate the assumption whereby all the received signals would originate from a single direction (i.e. the source of the counterfeit signal), which is done by mechanically generating variations of the radiation pattern by displacing its phase centre. This prior art, which will be detailed later, notably comprises the international patent application published under the number WO2014/047378. The mechanical antenna diversity generation devices are bulky and complex to drive and to process, which demands a high computation capability. They are therefore unlikely to be incorporated with mass market receivers, for example smart phones.
Now, the integrity constraints will rapidly extend to the terminals of this type if, as seems to be proving the case, their usage for semi-critical applications, such as transport or electronic payment, expands.
In the field of land transport, geolocation and guidance by GNSS system are rapidly being rolled out to the general public: personal vehicle navigation assistance, reservation of cycles or motor vehicles for rent, real-time tracking of urban transport, etc. The user can use either a specific GNSS signal processing module (for example installed in his or her personal vehicle), or his or her smart phone which incorporates geolocation functions based on GNSS signals, and/or on identification of the serving cell of the radio communication network or Wifi terminal, and mapping functions.
In the electronic payment field, the use of a mark of QRCode type or of a chip of NFC (Near Field Communications) type already allows for sight but contactless payment with a smart phone. The time-stamping and the certification of the place of the transaction are natural means for authenticating these payments, on condition of being able to guarantee the integrity of this time-stamping and of this location, which is not these days possible.